1. Field of the Invention
The present invention relates to high current electron sources having a narrow energy band such as are employed in electron beam printing devices, and particularly to such an electron source having a beam generating system consisting of a cathode and focusing electrodes and an anode wherein at least one electrode astigmatically focuses the electrons in the range of lower velocities in at least one crossover.
2. Description of the Prior Art
It has been experimentally shown that a narrow energy window or band of electrons generated by various emission processes such as, for example, thermal emission, Schottky emission or field emission, can only be achieved by the use of a very weak beam current. With increasing beam current, a spread of the energy distribution of the emitted electrons occurs which is a result of the Coulombian interaction of the electrons in the real crossover of the beam generating system and/or in the crossing areas occurring after the anode in the direction of electron beam flow. The sensitivity of the electrons to such Coulombian interactions is particularaly great in the cathode space where the velocity of the emitted electrons is still very small.
According to the calculations of H. Rose reported at the 19th Session of the Deutschen Gesellschaft fur Electronenmikroskopie of Sept. 9 through 14, 1979 in Tuebingen, the energy spread of a mono-energetic electron beam, that is, of an electron beam after the anode, is signficantly higher after traversing a stigmatic crossing point than after traversing an astigmatic crossover.
Moreover, it is also known to employ a grid electrode with a slot in the beam generating system in television tubes to reduce the energy spread in the real crossover of the beam, whereby the single crossing point which is present in the absence of the grid is replaced by two crossovers of low electron density. The positions and the dimensions of the crossovers are fixed by the size and the position of the slot in the grid. Such a technique is described in VDI Nachrichten No. 5, Feb. 2, 1979, page 8.